1. Field of the Invention
The present invention relates to an improvement in single-screw mechanisms of various types meant to vary the pressure of a fluid such as a liquid pump, gas compressor or expander, hydraulic motor, or the like.
2. Description of the Prior Art
The group of single-screw mechanisms which is of concern to the present invention are classified as positive-displacement rotary type machines. This invention relates to single-screw mechanisms that utilize, by way of example but not by way of limitation, planar, conical, cylindrical, toroidal, or other mainrotor shapes typically with cylindrical or planar gaterotors. In machines of this type, there is one mainrotor with a plurality of spiral threads that is driven by prime mover means so as to spin about a fixed axis within a fluid-tight stationary casing. There is at least one and usually two gaterotors, which are symmetrically disposed substantially transverse to the axis of the mainrotor, whose teeth penetrate through an opening in the machine's casing, called a window path, for meshing engagement with the threads of the mainrotor. The casing is provided with inlet and outlet ports for connecting the interior of this mechanism respectively to an intake and discharge plenum. The gaterotor teeth sweep the mainrotor threads drawing fluid into the mainrotor groove chamber from the inlet port and forcing the entrapped fluid from the thread groove into an outlet port provided in the casing. Sufficient torque is supplied by the prime mover means for rotation of the mainrotor towards the gaterotor tooth to overcome the discharge pressure being generated in the closed pocket of fluid defined in the groove chamber between the machine casing, mainrotor threads and gaterotor tooth.
Current practice in utilizing the single-screw mechanism as a pump or compressor has provided a single gaterotor tooth to seal off individual mainrotor grooves, thereby separating the higher pressure fluid from the intake side or inlet pressure. This single tooth must fit very closely to the mainrotor threads in order to minimize internal leakage and withstand the differential pressure forces applied to the opposite sides of the gaterotor tooth. Formerly, these factors have made it necessary to use a two part gaterotor comprising conforming nonmetallic gaterotor teeth each of which are backed by metallic supports on the low pressure side to provide adequate tooth stiffness. Supported gaterotor teeth are well known in the preceding art and are required in conventional single-screw mechanisms for operation at high differential pressures.
In the prior art, the inclusion of these metallic stiffeners has allowed the free flow of low pressure fluid between areas of the machine open to the inlet port and the gaterotor housing. This practice has persisted, especially where high outlet pressures are desired, in order to accommodate the use of the required metallic stiffener on the low pressure side of a nonmetallic gaterotor tooth. As these stiffeners are not conformant to the mainrotor thread, wide window pathways must be provided in the casing in order to permit mainrotor-gaterotor engagement. The presence of this nonconforming metallic stiffener thus permits a large area for the leakage of low pressure fluid leading from the low pressure side of the tooth and the low pressure inlet port into the gaterotor housing as can be seen in FIG. 3 at 14 and 15.
Problematically, in machines of the aforementioned design, unless the build up of fluid in the gaterotor housing is allowed to flow uninhibitedly back into the vicinity of the low pressure port, the pressure in the gaterotor housing will become elevated. Currently, gaterotor housing enclosures are designed to resist no more than low internal pressure, and as the inlet opening at 15 in FIG. 3 partially restricts fluid communication between the gaterotor housing and the inlet port area, present practice requires provision for "bleeding" off the fluid flowing into the gaterotor housing.
Numerous notable attempts have been made in the past to increase mechanical and volumetric efficiencies of these machines by limiting the presence of the many internal leakage paths that exist. Efforts to accomplish this end have included providing additional seals, reducing clearances, recapturing fluid leakage, changing mainrotor-gaterotor configurations and using various other techniques. By way of example, U.S. Pat. No. 4,105,378 reduces flow past the mainrotor band leakage path by locating a radially extending seal on the mainrotor closely adjacent to one end of the housing. U.S. Pat. No. 4,321,022 reduces flow past the gaterotor flank leakage path by utilizing gaterotor teeth flanks comprising at least three skewed surfaces which intersect on at least two edges so as to provide dual lines of sealing with the mainrotor thread. In U.S. Pat. No. 4,373,881, a conical mainrotor having a plurality of helical screw threads is engaged with a cylindrical gaterotor for increased outlet fluid volume by way of increasing the contact length and depth of each gaterotor tooth with each mainrotor groove. However, until recent developments have made it possible, sealing the gaterotor window leakage path has remained basically unaddressed except on the leading or high pressure side of the gaterotor teeth.
While some current single design configurations are capable of using self-supported gaterotors for low pressure applications (operation generally less than 150 psi outlet), common practice is to retain the use of gaterotor teeth stiffeners even when it unnecessary. However, several novel single-screw mechanisms have recently been introduced which obviate the need for metallic stiffeners where either high or low discharge pressures are required in a single stage. As exemplified in the co-pending U.S. Patent Application Ser. No. 07/287,352, filed Dec. 21, 1988, T. Bein discloses a single-screw compressor or expander comprising a compound conical mainrotor with cylindrical gaterotors wherein the inclusion of a hydrostatic type pressure port on the inlet side of the window path opening makes it possible to use a gaterotor tooth which has less or no need for additional structural support by way of backup stiffeners. In addition, the Inventor's U.S. Pat. No. 4,824,348, filed Aug. 27, 1986, discloses a multiple tooth engagement single-screw mechanism wherein several teeth are simultaneously engaged in each mainrotor thread. The advantage of this design is that it breaks down the high to low pressure gradient across two or more gaterotor teeth engaged in a single mainrotor thread groove and therefore allows use of gaterotor teeth which are self-supporting.